Oxidation of Molybdenum Disulfide Sheet in Water under in Situ

Apr 21, 2017 - Hubei Key Laboratory of Mineral Resources Processing and Environment,. Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei ...
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Oxidation of Molybdenum Disulfide Sheet in Water under in Situ Atomic Force Microscopy Observation Xian Zhang,† Feifei Jia,‡ Bingqiao Yang,§ and Shaoxian Song*,†,‡ †

School of Resources and Environmental Engineering and ‡Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China § School of Resource and Civil Engineering, Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, Hubei 430073, China ABSTRACT: In situ observation of molybdenum disulfide (MoS2) sheet exposed to water was performed using atomic force microscopy (AFM) to obtain a better understanding for the preparation and application of two-dimensional molybdenum disulfide. It was found that the MoS2 sheet underwent chemical reactions in water, leading to the partial etching of the surface layer and the formation of needlelike protrusions on the surfaces. X-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (AAS) analyses showed that this behavior might arise from the oxidation of MoS2 into Mo(VI) and SO42−. The oxidative product Mo(VI) was present in the form of either MoO3·H2O crystals on the MoS2 surface or molybdate ions in aqueous solution, whereas SO42− was present in the solution, resulting in the etching of the surface layer and the formation of needlelike protrusions. These findings might be significant in terms of applications of two-dimensional MoS2 and the concentration of molybdenite.

1. INTRODUCTION Two-dimensional molybdenum disulfide (MoS2), single or few layers of S−Mo−S units, has sparked widespread attention in recent years.1 The unit is referred to as monolayer MoS2, which consists of two hexagonal sheets of sulfur atoms and one intermediate hexagonal sheet of molybdenum atoms coordinated through ionic−covalent interactions with the S atoms in a trigonal prismatic arrangement.2 Monolayer MoS2 exhibits a 1.8 eV direct energy gap, leading to distinctive electronic, optical, and catalytic properties.3 In addition, it has an in-plane carrier mobility of 200−500 cm2/(V·s) and robust mechanical properties, making it an attractive material for flexible fieldeffect transistors (FETs).4,5 These superior properties are found only occurred in two-dimensional MoS2 with a perfect crystalline structure and are attenuated with increasing amounts of defects. On the other hand, MoS2 is easily oxidized, resulting in a large amount of crystalline defects.6,7 Therefore, it is of great significance to understand the stability of MoS2 exposed to different environments. Raman spectroscopy was used to determine the stability of MoS2 but failed to find any molybdenum oxide below 340 °C in an oxygen atmosphere.8 More sensitive technologies such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have also been used to study the oxidation of MoS2; they indicated that MoS2 or MoS2 films could be oxidized slowly when exposed to air and that the reaction was promoted with increasing humidity.9,10 Moreover, firstprinciples calculations based on density function theory (DFT) have been employed to demonstrate the interaction of MoS2 monolayers with oxygen.11−14 However, these © XXXX American Chemical Society

technologies have failed to provide a visible representation of the reactions. Atomic force microscopy (AFM) is a powerful microimaging technique for studying solid−liquid and solid−air interfaces.15,16 It allows for the direct and in situ nanoscale observation of solid surfaces, capturing the real-time changes of surfaces in their surrounding environments. Therefore, in situ observations of MoS2 by AFM might provide important information on the reaction of MoS2 in different environments, permitting great insight into the stability of MoS2. In this study, an attempt was made to study the oxidation of MoS2 sheets in water through in situ observations using AFM. The objective was to obtain direct evidence of the oxidation reaction and to provide visible observations of the morphology changes in the MoS2 surface during the oxidation process, to provide precautions for the application of MoS2 or twodimensional MoS2 in different environments. Meanwhile, XPS and atomic absorption spectroscopy (AAS) were used to analyze qualitatively the changes in chemical composition occurring during the oxidation of MoS2 in water.

2. EXPERIMENTAL SECTION 2.1. Materials. Single-crystal molybdenite collected from the Wuzhou Mine, Guangxi Province, China, was used in this work as molybdenum disulfide. The samples for AFM and XPS studies were of cuboid form with dimensions of 0.5 × 0.5 × 0.1 Received: February 25, 2017 Revised: April 8, 2017 Published: April 21, 2017 A

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NanoScope Analysis 1.5 software to assess the roughness and thickness of each sample. In the software, the images were flattened in second order without further processing. The dissolved Mo and S contents from molybdenum disulfide in water were determined using a ContrAA 700 Continuum Source AAS instrument (Analytik Jena, Jena, Germany). For each measurement, a 0.2-g powder sample was immersed in 4 mL of deionized water for 24 h, after which the suspension was filtered. The obtained liquid was collected to assay Mo and S. Chemical analysis with Ba2+ was used to confirm that the final oxidation product of S was SO42−. XPS spectra were collected with an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA) an Al Kα with X-ray source of 1486.6 eV under a vacuum chamber pressure of 5 × 10−7 mbar and for a measured spot size of 500 μm. The charge effects were calibrated by setting the C 1s photoemission at 284.2 eV.17 For the preparation of XPS samples, a fresh surface was first cleaved and then exposed to air or water for 4 days. It is necessary to make clear that the MoS2 samples were placed in air and that water was dropped on the fresh surface for sample exposure to water. The samples were measured without further treatment after the water was dried in the 4 days.

cm. The sample for AAS analysis was a powder with particle sizes in the range of 38−45 μm. Scanning electron microscopy (SEM) showed a smooth structure of molybdenite with some edgelike defects on the basal plane (Figure 1a). The energy-

3. RESULTS AND DISCUSSION Figure 2a−f shows AFM images (two-dimensional and threedimensional views) of a freshly cleaved MoS2 sheet before and after immersion in water. An atomically smooth surface of MoS2 was observed in air (Figure 2a). After the sample had been immersed in water for 1 h, needlelike protrusions in the range of 20−50 nm were observed on the surface of MoS2 (Figure 2b). More protrusions appeared as the immersion time increased to 12 h (as shown in Figure 2c−f). Figure 3 shows the roughness of the MoS2 surface after it had been immersed in water for different times. This figure shows that the roughness increased with increasing immersion time in water, indicating more protrusions might become loaded on the surface of MoS2. AFM imaging also demonstrated that the protrusions were needlelike at the beginning (Figure 2b), that some linked together to form short chainlike structures (Figure 2c−e), and that they finally came together into treelike structures (Figure 2f) as the number of protrusions increased. It should be pointed out that the height of these protrusions was about 1.5 nm and that no increase in height occurred as the immersion time increased. Another interesting finding presented in the AFM images is that MoS2 surface was etched slowly with increasing immersion time in water. The inset in Figure 2e shows the dimensions of the line marked in Figure 2e. It demonstrates that the thickness of the etched layer was about 0.6 nm, which is close to the layer-to-layer spacing of bulk MoS2.18,19 Therefore, it can be inferred that a monolayer of MoS2 was etched upon immersion in water. Notably, no additional surface etching was observed on the newly exposed MoS2 surface when the immersion time in water was extended . This is probably because the uniform monolayer coatings formed on the surface prevented the underneath layer from being etched.20 All of these experimental phenomena indicate that MoS2 cannot remain very stable in water. In addition, protrusions on the surface and etching of the surface layer occur when MoS2 is immersed in water, which will be explained later. XPS was performed to determine the chemical composition of the protrusions on the MoS2 surface upon immersion in

Figure 1. (a) SEM image and (b) EDS pattern of a freshly cleaved basal plane of a MoS2 sheet.

dispersive spectroscopy (EDS) pattern indicated that the studied sample was of high purity (Figure 1b). Therefore, no further treatment was used to purify the freshly cleaved molybdenum disulfide surface. Milli-Q water with a resistivity of 18.2 MΩ·cm and a pH of 5.65 was used in the experiments. 2.2. Measurements. A Quanta FEG 450 SEM instrument (FEI Company, Hillsboro, OR) was used to analyze the morphology of the molybdenite, and the elemental content of a sample was determined by energy-dispersive X-ray analysis. The molybdenite sample was cleaved to expose a fresh surface prior to experiments, and no further treatment was performed. In situ observation of MoS2 was performed using a MultiMode 8 AFM instrument (Bruker, Billerica, MA) in PeakForce tapping mode. A ScanAsyst-Air silicon nitride probe on a V-shaped cantilever (resonance frequency f 0 = 70 kHz, spring constant k = 0.4 N/m, dimensions of 115 × 25 × 0.65 μm) and ScanAsyst-fluid+ silicon nitride probe on a V-shaped cantilever (resonance frequency f 0 = 150 kHz, spring constant k = 0.7 N/m, dimensions of 70 × 10 × 0.6 μm) were used under air and liquid conditions, respectively. Each of the silicon nitride probes had a nominal tip radius of 2 nm. A fresh surface was cleaved with scotch tape prior to experiments. Images in air were captured under ambient conditions at 40% relative humidity and 25 °C with automatically optimized scan parameters including setpoint, feedback response, and scan rate. Images in water were captured by the following procedure: The sample with a fresh surface was mounted in the AFM liquid cell, and then 2 mL of deionized water was injected into the cell. All images were subsequently captured with 512 pixels and automatically optimized scan parameters for a predetermined time. The obtained images were analyzed with B

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Figure 3. Average roughness of the MoS2 surface after immersion in water for different times.

1s spectrum of MoS2 after exposure to water, which could be deconvoluted into four peaks. The peaks at 530.6, 531.6, 532.2, and 533.2 eV can be assigned to Mo−O from MoO3,21 hydroxide or hydroxyl groups (OH−) from MoO3·H2O or other hydrous forms of Mo(VI),22 absorbed H2O molecules,23 and O−C bonds,21 respectively. The Mo 3d XPS spectra of MoS2 exposed to air and water are provided in Figure 4c. Two characteristic peaks at about 229 eV [Mo(IV) 3d5/2] and 232 eV [Mo(IV) 3d3/2] could be observed upon MoS2 exposure either to air or to water.24 In addition, a new peak at 232.68 eV appeared upon MoS2 exposure to water, which corresponds to MoO3 emissions,24,25 indicating the formation of MoO3 on the MoS2 surface during exposure to water. This can also explain the presence of the peak corresponding to MoO3 at 530.6 eV in the O 1s spectrum (Figure 4b). Figure 4d presents the S 2p spectra of MoS2 exposed to air and water. The peaks with binding energies of about 163 and 162 eV can be attributed to the S 2p1/2 and S 2p3/2 orbitals, respectively, of negative divalent sulfide ions.26 No significant differences were observed after MoS2 was exposed to water, indicating that S atoms with other valences were not detected on the surface of MoS2. Therefore, the XPS results indicate that some Mo(IV) on MoS2 was oxidized to Mo(VI) upon exposure to water, leading to the formation of MoO3 on the MoS2 surface. AAS using a Continuum Source spectrometer was further performed to investigate the oxidation dissolution of MoS2 in water. The oxidation reaction of anisotropic molybdenite in water occurs primarily at the edges and defect sites in the basal plane.27 The defects on the basal surfaces of MoS2 consisting of intrinsic defects originated from its crystallization, and edgelike defects resulted from the cleavage,28,29 which could provide active sites for oxidation. Oxidation products under the same oxidation conditions were considered to be the same when oxidation occurred on the edge faces or basal planes of MoS2. By contrast, for molybdenite powder, obvious oxidation occurred at the edge faces of MoS2; therefore, molybdenite powder was chosen here for the better investigation of the molybdenum disulfide dissolution. Molybdenite powder (38− 45 μm) was first well dispersed in deionized water for 24 h, and then the water was collected to assay the Mo and S contents. AAS detected elemental contents of 28.5047 mg/L for Mo and 34.8856 mg/L for S in the aqueous solution, demonstrating that some dissoluble products were produced during the oxidation of MoS2. Figure 5a shows the Eh−pH diagram for molybdenum in water at room temperature. It indicates that Mo(VI) can be

Figure 2. AFM images of the fresh MoS2 surface before and after exposure to water: (a) freshly cleaved basal plane of MoS2 and (b−f) after immersion in water for (b) 1, (c) 2, (d) 3, (e) 5, and (f) 12 h.

water as observed in the AFM images. Figure 4a displays the wide-scan XPS spectra of MoS2 exposed to air and water. It indicates a significant increase in the oxygen content on MoS2 after exposure to water. Figure 4b further presents the XPS O C

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Figure 4. (a) Wide-scan XPS spectra of MoS2 exposed to air (bottom) and water (top). (b) High-resolution O 1s spectrum of MoS2 after exposure to water. (c) High-resolution Mo 3d spectrum of MoS2 exposed to air (bottom) and water (top). (d) High-resolution S 2p spectrum of MoS2 exposed to air (bottom) and water (top).

Figure 5. Eh−pH diagrams for (a) molybdenum and (b) sulfur in noncomplexing aqueous solution.

Figure 6. Schematic representation of the oxidation of MoS2 in water: (a) before oxidation, (b) after oxidation.

formed as MoO3·H2O in weakly acidic water. Therefore, the coatings formed on the surface of molybdenite observed in the AFM images might be MoO3·H2O crystals. Figure 5a indicates that Mo(VI) might also be present in the form of ions in aqueous solutions that are neutral or weakly acidic. Based on the combination of the Eh−pH and AAS results, it could be

deduced that molybdate ions, Mo7O246− for example, were produced as well during the oxidation of MoS2 in water.30 These findings agree well with the results of a previous work that the oxidation products of MoS2 precipitated from singlelayer MoS2 dispersions under acidic conditions could bind to the molybdenum disulfide surface or transfer into the D

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The Journal of Physical Chemistry C solution.31 According to previous works,32,33 S2− in sulfide minerals can be oxidized and be present in the form of sulfate in humid atmospheres. Therefore, S in the dissolved MoS2 solution detected by AAS might have resulted from SO42−. The Eh−pH diagram of sulfur in aqueous solution is shown in Figure 5b. It indicates that SO42− is the most stable form of sulfur in aqueous solution at room temperature. In addition, a small amount of white precipitate (BaSO4) was observed upon addition of Ba2+ to the aqueous solution, which further confirmed the oxidation of S2− in MoS2 to SO42−. Moreover, the pH of the aqueous solution decreased to 4.08 after the reaction, indicating the generation of H+ ions during the oxidation of MoS2. All of these experimental phenomena indicate that a chemical reaction occurred upon the exposure of MoS2 surfaces to water. From the XPS and AAS results, the surface oxidation of MoS2 might proceed according to the following equation in a weakly acidic water environment

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work from the National Natural Science Foundation of China under Projects 51474167 and 51674183 and the China Postdoctoral Science Foundation under Project 2016M600621 is gratefully acknowledged.

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MoS2 + O2 + H 2O → MoO3 · H 2O + Mo(VI)x Oy a − + SO4 2 − + H+

(1)

where Mo(VI)xOya− represents molybdate species containing Mo(VI), such as Mo7O246− and HMoO4−.30 Equation 1 can well explain the phenomena observed by AFM. The oxidation process of MoS2 in water is schematically illustrated in Figure 6. The MoS2 surface was smooth before being exposed to water, as shown in Figure 6a. However, the surface of the sample began to react with H2O and O2 in aqueous solution, resulting in the etching of the surface layer. As a result of the reaction, MoS2 was oxidized into MoO3·H2O or molybdate ions, and S was present in the form of SO42−. The oxidation product SO42− and molybdate ions would dissolve in the aqueous solution and be detected by AAS, whereas the MoO3·H2O crystals could remain on the surface of MoS2, thereby explaining the appearance of protrusions on the MoS2 surface (Figure 2b−f). The interesting finding from the AFM images is that the surface etching of MoS2 in water provides some precautions before the use of monolayer MoS2 in water. MoS2 is hydrophobic, whereas MoO3 is hydrophilic. Therefore, another finding is that the MoO3·H2O crystals formed on the surface of MoS2 might explain the change in the surface properties of MoS2 upon exposure to water.

4. CONCLUSIONS It was found that MoS2 is thermodynamically unstable when exposed in water, leading to the oxidation of MoS2 on the surface. This oxidation reaction produces Mo(VI) in the form of MoO3·H2O crystals on the surface and molybdate ions in aqueous solution and also produces SO42− dispersed in the aqueous solution, resulting in the partial etching of the surface layer and needlelike protrusions on the MoS2 surfaces. The oxidation of MoS2 in water might be detrimental to the application of MoS2, such as two-dimensional MoS2, and thus should be avoided.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaoxian Song: 0000-0001-7278-7875 E

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DOI: 10.1021/acs.jpcc.7b01863 J. Phys. Chem. C XXXX, XXX, XXX−XXX